The present invention relates to an image display tube comprising an envelope having a faceplate, a phosphor screen on or adjacent to the inner surface of the faceplate, means for generating a beam of electrons, a channel plate electron multiplier disposed adjacent to, but spaced from the phosphor screen, the electron multiplier comprising a plurality of discrete apertured dynodes arranged as a stack with the apertures in each dynode aligned with apertures in an adjacent dynode to provide channels, the apertures in the input dynode diverging in the direction of the incoming beam of electrons, and the maximum cross-sectional area of the apertures in the dynodes being substantially the same.
Electron multipliers have been proposed for image display tubes for example in British Patent Specification No. 1,434,053. In an image display tube a low energy electron beam produced for example by an electron gun is scanned across the input side of a large area channel plate electron multiplier which is disposed at a short distance from a phosphor screen provided on the inner surface of a substantially parallel faceplate. The electron beam undergoes amplification by current multiplication in the electron multiplier before being incident on the phosphor screen.
The channel plate electron multiplier comprises a stack of dynodes insulated from each other. Apertures in adjacent dynodes are aligned with each other to define channels. In use a substantially constant potential difference exists between adjacent dynodes. When a beam of electrons is incident on the input side of the channel plate electron multiplier, secondary electrons are produced of which the majority enter the channels and are multiplied so that an image is produced on the phosphor screen. Because the output is an image it is important to ensure that it is spatially correct to avoid distortions. Also it is desirable that the image should have good contrast and good brightness.
As approximately 24% of the area of a discrete dynode is occupied by apertures then it is inevitable that as an electron beam is scanned say in raster-like fashion across the input or first dynode that it will impinge on the dynode material between the apertures and produce secondary electrons. Some of these secondary electrons may enter a nearby channel but others may stray a relatively large distance across the input surface of the first dynode before entering a channel. Hence the image is degraded spatially and there is a corresponding reduction in contrast. If the cross-sectional area of each aperture is enlarged then this will lead to the overall structure being less rigid and therefore subject to the effects of vibration or, alternatively, if the number of enlarged cross-section channels is reduced to stiffen the dynodes then this is of no advantage in mitigating the problem of stray secondaries because the ratio of the area of the apertures to the area of the material between the apertures is returned towards that of the originally postulated situation. Furthermore channels of larger cross-sectional area will increase the possibility of incoming electrons passing through a channel without undergoing multiplication.
It has also been proposed to reduce the number of secondary electrons produced from the materials between the apertures by covering the material with a low secondary emitting material, such as carbon, having a secondary electron emission coefficient less than 2.0. While this improves the contrast it does not completely preclude the production of secondary electrons which may stray a relatively large distance before entering a channel.